Die casting component and method for the production thereof

ABSTRACT

The mechanical properties of a die casting component made of an aluminium alloy have the following proportions; magnesium 3.0-6.0 wt %, silicon 1.4-3.5 wt % manganese 0.5-2.0 wt %, iron max. 0.15 wt %, titanium max. 0.2 wt %, the remainder being aluminium and other components having a total proportion of a maximum of 0.2 wt %. Said mechanical properties improve significantly by virtue of the fact that the die casting component is subjected to heat treatment in the form of thermal hardening at a temperature T of 180° C.=T=320° C. over a period of at least thirty minutes after die casting.

The invention relates to a process for producing a pressure die-cast component from an aluminum alloy comprising the following constituents, in percent by weight Magnesium 3.0-6.0 Silicon 1.4-3.5 Manganese 0.5-2.0 Iron max. 0.15 Titanium max. 0.2 Remainder aluminum and other max. 0.2. components amounting to a total content of

The invention also relates to a pressure die-cast component produced using an aluminum alloy of this type.

It is known that the quality of pressure die-cast components depends on the one hand on the machine setting and the process parameters of the pressure die-casting operation, but on the other hand also on the aluminum alloy used, which has to be suitably adapted to the mechanical demands imposed on the pressure die-cast components.

To allow the production of aluminum pressure die-cast components which are weldable and have a high ductility, it is known to subject the pressure die-cast components to a heat treatment. The T6 heat treatment, which comprises a solution anneal with subsequent quench followed by hot age-hardening, is employed for the combination of a desired toughness, on the one hand, with an acceptable tensile strength and proof stress, on the other hand. This heat treatment causes distortion to the components, in particular during the solution anneal and during the quenching process, and consequently the components, in particular if they have been cast in thin-walled form, often require a further treatment.

EP 0 853 133 B1 has disclosed the process mentioned in the introduction, which was developed in order, by the use of the aluminum alloy indicated, to achieve an alloy microstructure which avoids the need for a heat treatment. Therefore, the advantage of the known alloy is regarded as being the fact that the components produced using it with desired mechanical properties do not require any subsequent heat treatment.

An AlMg5Si2Mn alloy of this type has a considerably higher tensile strength and a considerably higher proof stress as well as a considerably higher elongation at break than a conventional alloy AlMg5Si.

AlSi10Mg alloys which have been subjected to a T6 heat treatment comprising solution anneal, controlled quench and subsequent hot age-hardening are used to further improve the strength properties combined, at the same time, with a high plastic shape change capacity, i.e. good crash properties, in pressure die-cast aluminum components used on motor vehicles, with the drawbacks which result from distortion phenomena being accepted as the necessary accompaniment.

The invention is based on the object of allowing a pressure die-cast component which has better mechanical properties and can be produced without the need for subsequent treatments caused by distortion phenomena.

To achieve this object, according to the invention a process of the type described in the introduction is characterized in that the pressure die-cast component, after the die-casting operation, is subjected to a heat treatment in the form of hot age-hardening at a temperature T of 180° C.≦T≦320° C. for a period of at least half an hour.

Surprisingly, it has been discovered that pressure die-cast components which are produced using the aluminum alloy described, which is specially designed to eliminate a heat treatment, can have its mechanical properties considerably improved if the pressure die-cast components are subjected to hot age-hardening at a temperature over 180° C. The hot age-hardening at relatively mild temperatures, which are preferably between 220 and 280° C. and particularly preferably are approximately 250° C., do not entail any risk of distortion to the pressure die-cast component. However, extensive tests have revealed that in particular the proof stress (Rp_(0.2)) can be increased considerably by the hot age-hardening. The associated drop in the elongation at break is relatively minor and can be accepted, since in any event an elongation at break of >8%, which is sufficient for normal circumstances, is retained.

In a preferred alloy, the magnesium content is between 5.0 and 6.0% by weight and the silicon content is between 1.5 and 3.0% by weight. Trace contents of copper and zinc should not exceed 0.05% by weight and 0.10% by weight, respectively.

The hot age-hardening usually lasts for between half an hour and three hours, depending on the type of component, in particular the thickness of its walls.

In the text which follows, the technological properties of a pressure die-cast component produced in accordance with the invention are explained in more detail on the basis of comparative measurements illustrated in the drawing, in which:

FIG. 1 shows a diagram which shows the increase in the proof stress with elevated hot age-hardening temperatures,

FIG. 2 shows a diagram illustrating the dependency of the elongation at break values on the hot age-hardening temperature,

FIG. 3 shows a diagram illustrating the tensile strength and proof stress values for various alloys with and without a heat treatment,

FIG. 4 shows a diagram illustrating elongation at break values for the alloys used in FIG. 3.

In particular chassis parts for the automotive industry have to have high strength properties combined, at the same time, with a high plastic shape change capacity, since these parts must not break in the event of a crash.

The alloys used for these purposes are AlMg5Si2Mn (referred to below as “590”) or AlSi10Mg (referred to below as “360”), the components of the latter alloy being subjected to a T6 heat treatment. Consequently, the material is referred to below as “360 T6”.

The T6 heat treatment, which is composed of solution anneal, quench and hot age-hardening, not only incurs high costs but also conceals the risk of distortion to the parts. Even during casting itself, precautions have to be taken in order to minimize the gas content of the castings.

In the cast state, material 590 achieves proof stresses of approx. 180 MPa with an elongation at break of approx. 13%. Higher strength values are required for highly loaded parts and have not hitherto been achievable with this alloy, but these strength values can now be achieved by the heat treatment according to the invention in the form of hot age-hardening (T5 heat treatment).

FIG. 1 illustrates the increase in the 0.2% proof stress (Rp_(0.2)) as a function of the hot age-hardening temperature. Accordingly, in the cast state, the proof stress is approx. 165 MPa. The hot age-hardening at 180° C. allows this value to be increased to approx. 175 MPa. At a hot age-hardening temperature T of 210° C., the proof stress reaches almost 180 MPa. A sudden increase is observed at the preferred hot age-hardening temperature T=250° C., at which the proof stress rises to over 210 MPa.

FIG. 2 shows a drop in the elongation at break A, which characterizes the deformability, associated with the hot age-hardening. The abovementioned hot age-hardening produces a drop from the starting value of 13% to approx. 9%.

No influence of the hot-age hardening on the tensile strength (Rm) is observed.

FIG. 3 shows a comparison of the strength properties (tensile strength: rear values; proof stress: front values) for various aluminum alloys of types 226 (AlSi9Cu3 (Fe)), 260 (AlSi12 CuNiMg), 360, 360 T6, 590 (all comparison values) and 590 T5 (components according to the invention). It can be seen from this comparison that the strength properties of the material 590 T5 according to the invention are significantly superior to the strength properties of the material 360 T6, which has hitherto been regarded as optimum, even though the severe heat treatment T6, which constitutes a risk of distortion, is avoided. The material 590 T5 is also far superior to the material 360 T6 with regard to the deformability.

Since the mechanical-engineering properties of material 590 drop considerably as the wall thickness increases, it has not hitherto been possible to cast useable thick-walled parts from this material. The subsequent T5 heat treatment according to the invention allows the 0.2% proof stress (Rp_(0.2)) to be increased to such an extent that these parts no longer have to be produced from the material 360 T6.

The higher Rp_(0.2) values achieved according to the invention allow the material 590 T5 to be used even for parts which are subject to higher loads, e.g. as part of a welded structure with a wrought material.

The welding of the pressure die-cast components to metal sheets is expediently carried out prior to the T5 treatment. Weld seams normally constitute weak points. The weld metal usually consists of a mixture of cast alloy and wrought material. The properties correspond to those of the cast material. The hot age-hardening improves the strength properties of the weld metal.

Wrought alloys tend to recrystallize at higher temperatures, i.e. crystal grains of a different size and shape are formed from a deformed microstructure. If a certain temperature, known as the recrystallization threshold, is exceeded, the mechanical properties change very quickly. In the most unfavorable circumstances, coarse grains are formed and the strength values deteriorate.

The recrystallization threshold is influenced by alloying additions, in particular by the amount of Cr, Zr, Fe or Mn in dissolved or precipitated finely disperse form, by an annealing time and by the degree of cold-working, which lowers the recrystallization threshold.

The aim of recrystallization is to form a fine-grained microstructure. Accordingly, the metal sheet should have a degree of cold-working which is not too low (>30 up to 50%) and should be heated as quickly as possible to the recrystallization temperature.

In the case of an AlMg3 wrought alloy, as is typically used in the automotive industry, the critical temperature is approx. 250° C. This corresponds to the optimum hot age-hardening temperature of the process according to the invention.

The choice of the hot age-hardening temperature is of crucial importance with a view to avoiding the formation of coarse grain, with a decrease in the strength properties of the wrought material, while at the same time achieving the maximum possible increase in the 0.2% proof stress of the pressure die-cast component and of the weld metal. It is only by achieving higher proof stresses combined, at the same time, with high degrees of elongation and improved weld seam quality that the use of pressure die-cast components as part of lightweight structures is improved and in many cases made possible at all.

Higher hot age-hardening temperatures of over 250° C. are not ruled out in certain situations, in particular when using other wrought alloys or for specific applications. Pressure die-cast components produced in accordance with the invention, if a hot age-hardening temperature of 250° C. is used, can have a tensile strength Rm of >300 MPa, a proof stress Rp of ≧175 MPa and an elongation at break of >8%.

If the proof stress is over 200 MPa, it is possible to set an elongation at break of between 8 and 10%. If the proof stress is slightly lower, at ≧180 MPa, the elongation at break can be between 10 and 12%. 

1. A process for producing a pressure die-cast component from an aluminum alloy comprising the following constituents, in percent by weight Magnesium 3.0-6.0 Silicon 1.4-3.5 Manganese 0.5-2.0 Iron Max. 0.15 Titanium Max. 0.2 Remainder aluminum and other components max. 0.2 amounting to a total content of

characterized in that the pressure die-cast component, after the die-casting operation, is subjected to a heat treatment in the form of hot age-hardening at a temperature T of 180° C.≦T≦320° C. for a period of at least half an hour.
 2. The process as claimed in claim 1, characterized in that the magnesium contect is selected to be between 5.0 and 6.0% by weight.
 3. The process as claimed in claim 1, characterized in that the silicon content is selected to be between 1.5 and 3.0% by weight.
 4. The process as claimed in claim 1, characterized in that the maximum copper content is selected to be 0.05% by weight and the maximum zinc content is selected to be 0.10% by weight.
 5. The process as claimed in claim 1, characterized in that the hot-age-hardening is carried out at a temperature T of between 220 and 280° C., preferably at approximately 250° C.
 6. The process as claimed in claim 1, characterized in that the hot age-hardening is carried out for a period of between half an hour and three hours.
 7. The process as claimed in claim 1, characterized in that the pressure die-cast component, after the die-casting operation, is welded to wrought material and then subjected to the hot age-hardening.
 8. A pressure die-cast component made from an aluminum alloy comprising the following constituents, in percent by weight Magnesium 3.0-6.0 Silicon 1.4-3.5 Manganese 0.5-2.0 Iron Max. 0.15 Titanium Max. 0.2 Remainder aluminum and other components max. 0.2 amounting to a total content of

producible by the process as claimed in claim 1, characterized by a tensile strength (Rm) of >300 Mpa, a proof stress (Rp_(0.2)) of ≧180 Mpa and an elongation at break of >8%.
 9. The pressure die-cast component as claimed in claim 8, characterized by a proof stress of ≧200 Mpa with an elongation at break of between 8% and 10%.
 10. The pressure die-cast component as claimed in claim 8, characterized by a proof stress of ≧180 Mpa with an elongation at break of between 10% and 12%.
 11. The pressure die-cast component as claimed in claim 8, characterized by a magnesium content of between 5.0 and 6.0% by weight.
 12. The pressure die-cast component as claimed in claim 8, characterized by silicon content of between 2.0 and 2.5% by weight.
 13. The pressure die-cast component as claimed in claim 8, characterized by a maximum copper content of 0.05% by weight and a maximum zinc content of 0.10% by weight.
 14. The pressure die-cast component as claimed in claim 8, characterized in that it is constructed welded to a wrought material, which is consolidated together with the aluminum alloy by hot age-hardening. 